Temporally-decoherent and spatially-coherent vibrations in metal halide perovskite

TL;DR

This study reveals that in methylammonium lead iodide, optical phonons lose temporal coherence with temperature increase while maintaining spatial coherence, which may facilitate large polaron formation and enhance optoelectronic performance.

Contribution

It combines neutron scattering experiments with first-principles calculations to elucidate phonon coherence behavior in MHPs, providing new insights into polaron formation mechanisms.

Findings

Optical phonons lose temporal coherence at phase transition.

Spatial coherence of phonons persists despite decoherence.

Decoherent and coherent vibrations contribute to polaron formation.

Abstract

The long carrier lifetime and defect tolerance in metal halide perovskites (MHPs) are major contributors to the superb performance of MHP optoelectronic devices. Large polarons were reported to be responsible for the long carrier lifetime. Yet microscopic mechanisms of the large polaron formation including the so-called phonon melting, are still under debate. Here, time-of-flight (TOF) inelastic neutron scattering (INS) experiments and first-principles density-functional theory (DFT) calculations were employed to investigate the lattice vibrations (or phonon dynamics) in methylammonium lead iodide ($\rm{MAPbI_3}$), a prototypical example of MHPs. Our findings are that optical phonons lose temporal coherence gradually with increasing temperature which vanishes at the orthorhombic-to-tetragonal structural phase transition. Surprisingly, however, we found that the spatial coherence is still retained throughout the decoherence process. We argue that the temporally decoherent and spatially coherent vibrations contribute to the formation of large polarons in this metal halide perovskite.